Solvent-dependent thermodynamics of Iron(II), tris(2,2'-bipyridine) in saline solutions

Abstract

Introduction: The presented work evaluates intermolecular interactions and the solvation free energy between a ground and excited state of an iron complex in saline solution using MD simulations.
Background: MD simulations are utilized for estimating the solvation free energy in various systems and concentrations of salt. Any changes in the intermolecular interactions that might influence this are also examined.
Aim(s): This work aims to develop a method for such analysis using MD simulations in simple saline solutions. The method will be applicable in salt design, spectrochemistry, photocatalysis, among other fields.
Methods: Based on theoretical models and calculation methods, the free energy can be predicted with MD simulations. [Fe(bpy)3]2+ spin states are modelled using partial charges, which are solvated in solutions of 0-1 M NaCl. Finally, thermodynamic integration is used for determining the free energy. Radial distribution functions are used to examine any changes in solvent/ion distributions and preferential interactions using Kirkwood-Buff integrals.
Results: The free energy differs only little between the LS and HS state but induces an ion redistribution. A change in preferential interaction depending on the complex spin state as well as salt concentration is also observed.
Conclusion: Using the presented method, both solvation free energy and preferential interactions can be evaluated. However, as the results indicate, the change in free energy between spin states depends on the set of partial charges that is used. Factors such as geometrical changes in the complex may have a significant impact that needs to be considered.

Popular Abstract

The salt effect – Complex interactions and the modelling of free energy.

Particles and molecules surround us everywhere we go. They make out the air we breathe, the things we eat, the things we touch. Some molecules have the ability to extract energy from light, such as chlorophyll in plants. This is possible as chlorophyll contains atoms that can be excited by the waves of light from the sun. Other molecules may have similar light-absorbing properties.

Light is energy once absorbed by a molecule, creating an excitation. This often rearranges electrons and can cause geometrical changes in the molecule. However, such states are not maintained forever. The molecule eventually “relaxes” back to its initial ground state. This releases the excess (free) energy to the surroundings, which can be used to perform a chemical reaction for example. In plants, this energy is used to create energy carriers (called ATP).

Beside the intramolecular properties, such processes are influenced by the environment like an aqueous solution. Water is “polar” since there is an uneven charge distribution in the molecule, while other solvents may be nonpolar. Polarity impacts how the solvent and a molecule in solution interact electrostatically and can stabilize and influence states. Saline solutions contain ions – uniformly charged particles – that may further enhance or mitigate such processes like polar solvents.

Examining light-induced changes in a system may be very time-consuming and difficult to understand. Instead, one can turn to Molecular Dynamics (MD) simulations. MD simulations essentially enables the user to perform experiments in a computer, which can reveal chemical occurrences in full atomic detail. This allows testing of hypotheses and predicting experimental outcomes, bridging theory and practice. To gain insight into how the solvent composition influences the described states of a molecule, MD simulations can be used.

In this work we examine the ground and excited states of an iron-centered metal complex in water. The states are mimicked and derived from quantum mechanical calculations and used in MD simulations, and the free energy and interactions between molecules are evaluated. To examine its impact, solutions with and without salt were simulated.

The simulations could successfully estimate the free energy difference and connect it to interaction changes using the given constraints. The free energy between the ground and excited state was seemingly very small for the iron complex, and adding salt had no significant impact. This could possibly be due to an important change in the iron complex geometry being absent in the simulation, as well as a minute dipole change between the states. However, interactions between the iron complex, solvent and the ions differ, indicating that salt could be used for stabilizing certain molecular states.

• author: Ohlson, Oliver
• supervisor: Mikael Lund
• organization: Department of Chemistry
• course: KEMR45 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: Physical chemistry, Theoretical chemistry, MD simulations, intermolecular interactions, solvation free energy, spin state, saline solutions
• language: English
• id: 9137109
• date added to LUP: 2023-09-11 13:17:49
• date last changed: 2023-09-11 13:17:49